In recent years, many different inventions have been proposed to detect and compute the rotation and position of a listener's head wearing headphones. One class of inventions proposes a transmitter which is mounted on the headphones. It is used for emitting a signal only, from which the head position and orientation can be extracted. The transmitted signal is either based on infrared, ultrasonic, or magnetic-field signals. A corresponding receiver will feed the received signal to a signal-processing unit where the rotation and/or position of the listener's head is calculated. Such device is disclosed in WO 92/07346.
Another class of inventions propose a gyrator or an angular velocity device, which is mounted on the headphone. In this case, the rotation of the head can be acquired just from the angular velocity device without any other receiver device. Such devices are used for example in digital cameras where hand movements are compensated. However, the drift of angular velocity devices can be significant. Therefore, this method is preferred for measuring relative motions only. Such device is disclosed in EP-1 176 848 A2.
A third class of inventions is a combination of the latter two methods. Therefore, an angular velocity device is used for measuring the head movements, whereas an additional ultrasonic or infrared transmitter and receiver are used to calibrate the drift of the angular velocity device.
Different methods also exist for tracking the listener's position in a room with loudspeakers. The most common method is to reproduce an audible signal using the loudspeakers and measure the delay using one or several microphones. The position of the listener can be computed from the known position of the loudspeakers and the measured delay. Until now, all methods and devices in that field measure the listener's position once before sound reproduction starts. After that, a fixed sweet spot exists where a listener has the best sound experience. Changing the sweet spot requires to stop the sound reproduction and to perform a new measurement.
JP 06 082242 discloses a device for calculating the position and orientation of a headphone in space using ultrasonic signals. In addition to the transducers used for the acoustical signal, three more transducers are fixed on the headphone for treating ultrasonic signals used for defining the position and orientation in space. Further three transducers are positioned in space for receiving ultrasonic signals. In this case a electrical or optical reference signal is transmitted from the headphone to the receivers of the ultrasonic signals and the delays of each ultrasonic signal is calculated and its emitter is determined. The reference signals are separated by using different frequency bandwidths. One drawback of this apparatus is to be seen in the fact that special transducers for determining the position and orientation are necessary in addition to the transducers used for acoustical signals. Also an electrical or optical reference signal has to be provided. This arrangement and the division of the ultrasonic signals into several frequency bands leads to a very sophisticated method for the treatment of the signals in order to determine the position and orientation of the headphone.
JP 01 276900 discloses a device with loudspeakers dispersed in a space and whereas the position of a listener should be determined. Therefore, the loudspeakers emit ultrasonic signals which are received by a receiver located on the listeners position. Starting from the different running times needed by said ultrasonic signals to reach the receivers, the position of the receiver or the listener can be determined. As no separate ultrasonic transducers are provided, the step of determining the position must be performed ahead of emitting the acoustical signal, that is the effective operation of the loudspeakers. The loudspeakers are in a fixed position and the position is therefore determined once only in advance to the operation of the loudspeakers. The position of the listeners head can not be measured continuously.
Both devices take advantage of correlating incoming ultrasonic signals to determine the position. That means that said devices use the signals having the most important amplitudes, but, such signals are often those who are reflected by other objects. For this reason, the position determined by such devices is heavily influenced in a negative manner by external conditions.